Abstract 205: Myocardial Expression of Murf-1 and MAFbx After Induction of Heart Failure: Effect on Myocardial Contractility
Background: In chronic heart failure (CHF) the myocardial expression of the inflammatory cytokine TNF-α, which is thought to contribute to myocardial remodeling, was found to be increased. However, it is unknown, whether the E3-ubiquitin ligases MAFbx and Murf-1 are involved in this remodeling process and whether their expression is regulated by TNF-α.
Methods: Rats underwent LAD-ligation to induce CHF or were sham-operated (con). The expression of MAFbx/Murf-1 and troponin I was analyzed by RT-PCR and Western-blot in the non-infarcted area of the left ventricle. In cell culture experiments the potency of TNF-α to stimulate Murf-1/MAFbx expression, the intracellular signaling pathway and the involvement of the E3-ligases for the impairment of contractility was assessed.
Results: In CHF the myocardial expression of TNF-α was elevated 3.1-fold as compared to control (con: 325±42 vs. CHF: 1027±129 pg/mg; p>0.0001). This was associated with a 4.5-fold and 2.7-fold increase in MAFbx (con: 13.2±2.3 vs. CHF: 60.5±11.4 arb. units; p>0.001) and Murf-1 (con: 2.1±0.4 vs. CHF: 5.6±0.9 arb. units; p>0.01) expression, respectively. A positive correlation between TNF-α and the expression of MAFbx (r=0.82; p<0.05) or Murf-1 (r=0.75; p<0.05) was evident. In addition, the expression of the contractile protein troponin I was significantly decreased in CHF (con: 1.6±0.3 vs. CHF: 0.8±0.1 arb. units; p>0.05) and a close correlation between Murf-1 protein and troponin I expression was evident (r=0.52; p<0.05). In neonatal rat cardiomyocytes, TNF-α induced the expression of MAFbx through p38MAPK dependent pathways, whereas the induction of Murf-1 required the activation of the p42/44 MAPK pathway. Exposure of cardiomyocytes to TNF-α resulted in troponin I ubiquitination, subsequent degradation and a decline in contractility. This was completely abrogated by siRNAs against Murf-1/MAFbx.
Conclusion: TNF-α, which is increasingly expressed in CHF induces troponin I degradation through a MAFbx/Murf-1-dependent pathway. This was associated with an impairment of contractility and might be one mechanism involved in the adverse remodeling process in CHF.